This invention is concerned with a method for treating cyanide laden waste water effluents derived from coke-oven operations, blast furnace blowdown and other industrial wastewater.
The treatment of aqueous effluents from coke oven operations for discharge to receiving bodies without harm to the environment is a major problem facing the steel industry. These wastes are complex in composition, time-varying in rate of production and constitute one of the major pollution sources associated with steel-making. The waste liquors from coking operations, for example, contain many of the by-products of the pyrolysis reactions which occur during the high temperature carbonization of coal. From the standpoint of water pollution, it is necessary to greatly reduce the contaminants contained in such streams, particularly phenols and cyanides, free and fixed ammonia, free and emulsified oils, and various suspended solids, including sulfides. The treatment and removal of such residual contaminants involves sophisticated chemical techniques and high costs.
In the past, various process schemes have evolved for recovering and treating the volatile products of coking operations. For the high temperature carbonization process used to produce metallurgical coke in slot-type ovens, for example, the gaseous mixture leaving the ovens consists of dust, permanent gases, e.g., hydrogen, methane, carbon monoxide, carbon dioxide, nitrogen etc., having a fuel value of about 550 Btu/cubic foot, together with vapors which are condensed and separated to produce crude tar, light oil (benzol) and ammonia liquor. The latter products are then further processed and refined to produce a myriad of chemicals for commercial sale.
In one of the most generally used recovery systems, the gas leaving the coke ovens is first contacted with continuously recirculated aqueous sprays which cool the gas from about 1500.degree. to 185.degree. F. This condenses most of the tar which is subsequently decanted from the cooling water. The gas is then further cooled to 75.degree.-95.degree. F by further washing with ammonia liquor to condense more tar which is then separated and recovered. Final tar removal (tar fog) is accomplished by passage through electrostatic precipitators.
The next step in the gas treatment after tar removal is the recovery of ammonia. This is usually accomplished by contacting the gases from the tar separation step with a solution containing 5% free sulfuric acid. Crystals of ammonium sulfate which form in the contacting step are separated by centrifuging, dried and then sold for use as a fertilizer component. After sulfate removal the residual aqueous stream is further stripped of ammonia by steam distillation in an ammonia still. Steam distillation makes it possible to further reduce the ammonia level in this stream. The additional ammonia thus recovered is recirculated to the sulfuric acid scrubbers and recovered as ammonium sulfate.
Other methods of ammonia recovery can be employed such as washing the gas with water in a series of scrubbers, mixing the liquor so-formed with other liquor from the collecting mains and distilling. Still other methods employ phosphoric acid in the scrubbers to produce mono- or di-ammonium phosphate or absorb the vapors from the ammonia still in water to form a 15-25% aqueous solution of ammonia. Regardless of which ammonia recovery method is used, a final aqueous weak ammonia liquor (WAL) waste is formed which must be treated prior to final discharge to a receiving body.
That portion of the ammonia which exists as the salt of a weak acid is considered "free" and is liberated by simple boiling during distillation in the ammonia still while the "fixed" ammonia existing as chlorides or thiocyanates requires liberation by the addition of lime before it can be recovered by distillation. Accurate control of the lime slurry addition in the "lime leg" of the ammonia still is of critical importance in achieving a low ammonia containing effluent.
The next step in gas processing is light oil recovery which is accomplished by further cooling the gases from the ammonia scrubber with water to 70.degree.-75.degree. F. This results in the condensation of napthalene which can be removed as crystals from the wash water or by various solution and distillation techniques using selected petroleum fractions or solvents. The crude light oil containing benzene, toluene and xylene is recovered by contact with a petroleum wash oil in packed towers. The wash oil is then stripped with steam. The aqueous waste streams from these operations also contain appreciable amounts of contaminants and are usually mixed with the WAL prior to treatment.
The coke oven gas generally is given no further treatment and is used as a fuel for various purposes in the steel mills. In some cases, however, hydrogen sulfide is removed. A variety of methods can be used such as absorption by sodium carbonate-sodium bicarbonate solution followed by stripping under vacuum (vacuum carbonate process). The resultant sulfides are converted to elemental sulfur in a Claus kiln.
In summary, the treatment of the gaseous pyrolysis products resulting from the high temperature carbonization of coal to produce metallurgical coke generates three principal aqueous waste streams. These streams result from treatments to cover tar, ammonia, and light oils from the coke oven gas. The amount and composition of the contaminants in these waste streams varies with the type of coal blend being coked, the operating conditions for coking and the specific processes used to recover the tar, ammonia and light oils. As a result, after these streams are mixed it is difficult to generalize with respect to the composition of the aqueous waste stream that has to have further treatment before discharge to receiving bodies or to municipal sewage plants.
In general, the weak ammonia liquor (WAL) obtained after steam distillation of the rich ammonia liquor freed from tar and ammonium sulfate and which has been combined with the waste water contaminants after the light oil removal steps contains a large number of materials in small concentration such as ammonia, cyanides, phenols, oil and grease, thiocyanates, and suspended solids. Table I below illustrates a typical weak ammonia liquor composition before and after conventional treatments to remove contaminants contained therein.
TABLE I ______________________________________ A B Combined Aqueous Combined Aqueous Components Bio-Oxidation Bio-Oxidation (ppm) (ppm) (ppm) ______________________________________ pH 8.5 6.5 BOD.sup.1 2000 200 COD.sup.2 2570 455 TOC.sup.3 865 225 NH.sub.3 (total) 55 65 NH.sub.3 (free) 4 8 NH.sub.3 (fixed) 51 57 Cyanide (total) 8 5 (simple) 4 1 (complexed) 4 4 Oil and grease 25 26 Phenols 440 &lt;0.25 Suspended Solids 245 100 Thiocyanate 200 10 ______________________________________ .sup.1 Biological Oxygen Demand .sup.2 Chemical Oxygen Demand .sup.3 Total Organic Carbon
The cyanides are present in the form of simple cyanides (CN).sup.-- and metal complexed cyanides. The complexed cyanides include ionized cyanides in solution such as Fe(CN).sub.6.sup.4-, [Fe(CN).sub.5.H.sub.2 O].sup.3-, and FeFe(CN).sub.6.sup.-1 ; charged colloidal cyanides such as [FeFe(CN).sub.6 FeFe(CN).sub.6 ].sup.2- and solid particulate cyanides such as Fe (II) [Fe(CN).sub.6 Fe] and Fe(II) [Fe(CN).sub.6 Fe].sub.2. The cyanides may also be complexed with other cyanide-complexing metals although iron is the predominant metal in weak ammonia liquors. The type of complexed cyanide formed depends on the Fe/CN ratio. Generally as this ratio increases, more solid particulate cyanides are formed and as the ratio decreases, more ionized and charged colloidal cyanides are formed. Thiocyanates are also present in simple and complexed form. For purposes of brevity, the above mentioned cyanides and thiocyanates, in either simple or complexed form will be referred to as cyanide-containing compounds.
The "free" ammonia exists as the salt of a weak acid and the "fixed" ammonia exists as the chloride or thiocyanate as previously mentioned.
These weak ammonia liquors are treated prior to discharge into receiving bodies such as sewage systems by first introducing the liquors into a settling basin to remove suspended solids and then after clarification removing the phenols by bio-oxidation with special bacterial strains acclimated to phenols (see column A and B of Table I). The cyanide-containing compounds, especially the simple cyanides, are the most toxic materials present after bio-oxidation and are also the most difficult to remove. The removal of the cyanide-containing compounds and the remaining organics and ammonia is presently accomplished by chlorination which produces various toxic reaction products such as chloramines and chlorinated hydrocarbons. Chloramines adversely affect waste water quality and kill fish. Many chlorinated hydrocarbons are either known or suspected carcinogens. Thus the chlorination step merely converts one toxic material into another.